Sending e-sign document via Apex Trigger or Process Builder

Hi,

I want to send e-sign agreement either with an Apex trigger or via Process Builder. I have configured "Send on behalf of" field to a salesforce user. I have created a record in echosign_dev1__SIGN_Agreement__c object, have populated recipient and linked to master opportunity as well as follow

for(Opportunity opp: opps)

    {

        if(opp.Offer_Stage_Status__c=='Offer Accepted' && opp.Applicant_1__r.PersonEmail!=null)

        {

            echosign_dev1__SIGN_Agreement__c ag=new echosign_dev1__SIGN_Agreement__c();

            ag.Name=opp.Name + System.now();

            ag.echosign_dev1__Sender_User__c=setting.EchoSign_Sender_User__c;

            ag.echosign_dev1__Opportunity__c=opp.Id;

            ag.echosign_dev1__Recipient__c=opp.Applicant_1__r.PersonContactId;

            agreements.add(ag);

            mapOppAgreement.put(opp.Id, ag);

        }

    }

    if(agreements.size()>0)

        INSERT agreements;

The above code inserts the agreements. There is more code after the above segment which attaches a PDF file to the record inserted above.

Now I want to trigger the send action. Any idea how should I trigger the document sending?

Ideally I'd like API function to send the document.

Regards,

Attiq